The present disclosure relates generally to switching-mode power supplies, and more particularly to switching-mode power supplies capable of performing valley switching.
A switching-mode power supply normally uses s power switch to control the current through an inductive device, such as an inductor or a transformer. In comparison with other types of power supplies, switching-mode power supplies generally benefit from compact product sizes and efficient power conversions, and are adopted by most of power supply manufactures.
Among a variety of switching-mode power supplies, some can operate in a quasi-resonance (QR) mode and are named as QR-mode power supplies. A QR-mode power supply could perform zero-voltage switching, meaning a power switch is turned ON substantially at the moment when a voltage across the power switch is zero or minimum, so switching loss of the power switch could be minimized in theory. A QR-mode power supply generally enjoys excellent power conversion especially when the load it drives is heavy.
FIG. 1 shows a QR-mode power supply 10 in the art, where a transformer is an inductive device with a primary winding PRM, a secondary winding SEC and an auxiliary winding AUX, inductively coupled to one another. The QR-mode power supply 10 converts the input voltage VIN into an output voltage VOUT and an output current IOUT to power a load 24. A QR power controller 26 provides at a node GATE pulse-width modulation (PWM) signal VGATE to periodically turn ON and OFF a power switch 34. Via voltage-dividing resistors 28 and 30, the QR power controller 26 detects winding voltage VAUX across the auxiliary winding AUX. FIG. 2 shows waveforms of PWM signal VGATE, winding voltage VAUX, valley signal SQRD and blanking signal SBLANK. In FIG. 2, between two rising edges of the PWM signal VGATE is one cycle time TCYC consisting of one ON time TON and one OFF time TOFF, where an ON time TON is a period of time when the power switch 34 is turned ON, and an OFF time TOFF, in the opposite, is a period of time when the power switch 34 is turned OFF. As shown by the rear half of an OFF time TOFF in FIG. 2, because of the completion of demagnetization, the winding voltage VAUX starts oscillating to generate two signal valleys VL1 and VL2. The QR power controller 26 provides a valley signal SQRD which pulses each time when a signal valley is detected. The QR power controller 26 concludes the cycle time TCYC based upon the occurrence of the signal valley VL2, and this kind of switching ON a power switch in response to the occurrence of a signal valley is commonly called as valley switching.
At a compensation node COMP of the QR-mode power supply 10 is a compensation voltage VCOMP controlled by an operational amplifier 20 comparing the output voltage VOUT with a target voltage VTAR. The compensation voltage VCOMP substantially determines the durations of the ON time TON and a blanking time TBLANK, and the power controller 26 allows the cycle time TCYC to end after the blanking time TBLANK. The blanking time TBLANK can avoid the QR-mode power supply 10 from suffering low power conversion because of a too-high switching frequency fCYC (=1/TCYC). As demonstrated in FIG. 2, the blanking time TBLANK continues and does not end when the signal valley VL1 appears, so the power switch 34 is still turned OFF. Nevertheless, the signal valley VL2 appears when the blanking time TBLANK has concluded, and it makes the power switch 34 turned ON to start a next cycle time. A signal valley selected to perform valley switching is called a selected valley hereinafter, and in FIG. 2 it is the signal valley VL2.
The QR-mode power supply 10 in the art could generate uncomfortable audible noise, however. For a constant load 24, the compensation voltage VCOMP could vibrate, making the selected valley jump to and fro between two adjacent signal valleys, and resulting in an instable operation and noise in the transformer. Since a power supply operating with noise is hardly accepted in the market, the noise caused by the valley jump should be diminished or eliminated.